Russia has warned the United States against taking "hasty and reckless" steps over Moscow's military intervention in Crimea.

In a telephone conversation with US secretary of state John Kerry, Russia's foreign minister Sergei Lavrov said any sanctions imposed would boomerang back on the United States, Russia's foreign ministry said.

Mr Kerry stressed the importance of resolving the situation through diplomacy and said he and Mr Lavrov would continue to consult, the State Department said.

It was the second tense, high-level exchange between the former Cold War foes in 24 hours over the pro-Russian takeover of Ukraine's Crimean peninsula.

It comes after Russian president Vladimir Putin rebuffed a warning from US president Barack Obama.

After an hour-long telephone call on Thursday, Mr Putin said Moscow and Washington remain far apart on the situation in the former Soviet republic.

"Russia cannot ignore calls for help and it acts accordingly, in full compliance with international law," he said in a statement.

Mr Putin said the new authorities in Ukraine had taken "absolutely illegitimate decisions on the eastern, south-eastern and Crimea regions".

Ukraine's border guards said Moscow had poured troops into the southern peninsula where Russian forces have seized control.

There are now 30,000 Russian soldiers in Crimea, said Serhiy Astakhov, an aide to the border guards' commander.

Before the crisis there were only 11,000 soldiers based with the Russian Black Sea fleet in the port of Sevastopol, he said.

Mr Putin denies that the forces, which have no national insignia, surrounding Ukrainian troops in their bases are under Moscow's command, although their vehicles have Russian military plates.

The West has ridiculed his assertion.

Referendum 'illegitimate and will violate Ukraine constitution'

The most serious east-west confrontation since the end of the Cold War escalated on Thursday, when Crimea's parliament, dominated by ethnic Russians, voted to join Russia.

European Union leaders and Mr Obama denounced the referendum as illegitimate, saying it would violate Ukraine's constitution.

Ukrainian prime minister Arseniy Yatsenyuk said no-one in the civilised world would recognise the result of the "so-called referendum".

He repeated Kiev's willingness to negotiate with Russia if Moscow pulls its additional troops out of Crimea.

But the head of Russia's upper house of parliament said that Crimea had a right to self-determination and ruled out any risk of war between "the two brotherly nations".

The last time Crimeans were asked, in 1991, they narrowly voted for independence along with the rest of Ukraine.

Meanwhile, Mr Obama announced the first sanctions against Russia since the start of the crisis, ordering visa bans and asset freezes against people deemed responsible for threatening Ukraine's sovereignty.

Russia has earlier warned it will retaliate against any sanctions.

Japan endorsed the Western position.

But China - often a Russian ally in blocking Western moves in the UN Security Council - was more cautious and said sanctions were not the best way to solve the crisis.

Brussels and Washington rushed to strengthen the new authorities in Ukraine, announcing both political and financial assistance.

The regional director of the International Monetary Fund said talks with Kiev on a loan agreement were going well and praised the new government's openness to economic reform and transparency.

The European Commission has said Ukraine could receive nearly $17 billion in the next couple of years provided it reaches agreement with the IMF, which requires economic reforms like ending gas subsidies.